[swift-users] function advancedBy() bugs

[zh ao]

Xcode Version 7.3.1 (7D1014), swift 2.2 along with Xcode.

Zhaoxin

On Tue, May 17, 2016 at 8:46 PM, zh ao <owenzx at gmail.com> wrote:

> It seams that I encountered bugs in function advancedBy().
>
> //: Playground - noun: a place where people can play
>
>
> import Foundation
>
>
> do {
>
>     var str = "abcdefg"
>
>     var range = str.startIndex..<str.endIndex // 0..<7
>
>     str += "hijklmn"
>
>     range.endIndex = str.endIndex // 0..<14
>
>     let index = range.startIndex.advancedBy(10)
>
>     //fatal error: cannot increment endIndex
>
> }
>
>
> do {
>
>     var str = "abcdefg"
>
>     str += "hijklmn"
>
>
>
>     var range = str.startIndex..<str.endIndex // 0..<14
>
>     let index = range.startIndex.advancedBy(10)
>
>     range // 0..<14
>
> }
>
>
> do {
>
>     var range = 0..<7
>
>     let index = range.startIndex.advancedBy(10)
>
>     range // 0..<7
>
> }
>
>
> There are three do blocks. All codes are similar.
>
>
> First block and second block is the almost same. But I encounter an error
> in the first block, saying "fatal error: cannot increment endIndex". It
> shouldn't appear as the endIndex of range is 14 instead 7.
>
>
> Second block and third block both used function advancedBy, however, in
> the second block, the method is from BidirectionalIndexType,
>
>
> extension BidirectionalIndexType {
>
>     @warn_unused_result
>
>     public func advancedBy(n: Self.Distance) -> Self
>
>     @warn_unused_result
>
>     public func advancedBy(n: Self.Distance, limit: Self) -> Self
>
> }
>
>
> ​in the ​third block, it is RandomAccessIndexType,
>
>
> extension Int : RandomAccessIndexType {
>
>     /// Returns the next consecutive value after `self`.
>
>     ///
>
>     /// - Requires: The next value is representable.
>
>     public func successor() -> Int
>
>     /// Returns the previous consecutive value before `self`.
>
>     ///
>
>     /// - Requires: The previous value is representable.
>
>     public func predecessor() -> Int
>
>     public func distanceTo(other: Int) -> Distance
>
>     public func advancedBy(n: Distance) -> Int
>
> }
>
>
> ​So the range in the third block after advancedBy is unchanged. It makes
> sense here. But I think people may feel strange as one is a mutating
> function and the other is not.
>
> ​Zh​aoxin
>
>
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